Rain can indeed freeze even when the air temperature measured by a thermometer reads above the freezing point, such as \(36^\circ\text{F}\). The process is a complex interaction of atmospheric physics and surface thermodynamics, not simply a matter of the surrounding air temperature. Freezing rain, which is rain that turns to ice on impact, occurs because the temperature of the surface the rain is hitting is at or below \(32^\circ\text{F}\) (\(0^\circ\text{C}\)). This phenomenon involves a chain of events where the ground or an object can lose heat more efficiently than the air directly above it, setting the stage for dangerous icing conditions.
The Critical Difference: Air Temperature vs. Surface Temperature
The temperature reported by weather services is the air temperature, typically measured several feet above the ground in a sheltered location. This air temperature often differs significantly from the surface temperature of objects like roads, sidewalks, and car windshields. The freezing process is governed by the temperature of the contact surface, not the air the rain falls through.
Surfaces lose heat through a process called radiative cooling, where objects continuously emit infrared energy into the atmosphere and space. On clear nights, especially, pavement and other materials radiate more heat than they receive, causing their temperature to drop below the ambient air temperature. Surfaces that lack high thermal inertia, or the ability to retain heat, cool down much more rapidly than the air mass.
This difference is particularly noticeable on elevated structures like bridges and overpasses. Because these structures have cold air circulating both above and below them, they are exposed to cooling on all sides. This extensive exposure allows them to lose heat quickly and maintain a temperature at or below freezing, even when the air temperature remains a few degrees above \(32^\circ\text{F}\). The rain may be \(36^\circ\text{F}\) in the atmosphere, but upon striking a \(30^\circ\text{F}\) bridge deck, it instantly turns to ice.
Supercooling: When Liquid Water Drops Below Freezing
The rain that causes freezing on impact is often in a state known as supercooled liquid water. This is liquid water that has cooled below its normal freezing point of \(32^\circ\text{F}\) but has not yet solidified into ice. This peculiar state is possible because water requires a nucleation site—a tiny impurity, dust particle, or surface—to provide a template for the ice crystal structure to form.
Raindrops that fall through a shallow layer of sub-freezing air near the surface often lack these internal impurities. Without a surface to nucleate the ice, the water molecules cannot properly align to form a solid crystal lattice. The droplets remain suspended as liquid, even as their temperature drops a few degrees below the freezing point.
This supercooled liquid state is highly unstable. When a droplet encounters a solid object that is also at or below freezing, the cold surface acts as the necessary nucleation site. The impact and contact immediately trigger the solidification process, and the droplet freezes instantly onto the surface in a process known as accretion. The result is a sheet of clear, hard glaze ice.
Defining Types of Icy Precipitation
The term “icy precipitation” covers several forms, with freezing rain and sleet being the most common in this temperature range. They are differentiated by their formation process. Freezing rain occurs when the air temperature is slightly above freezing, but the surface is colder, causing the supercooled liquid to freeze upon contact.
Sleet, in contrast, consists of small, translucent ice pellets that bounce when they hit the ground. Sleet forms when a snowflake melts into a raindrop as it passes through a warm layer of air aloft, then encounters a deep layer of sub-freezing air closer to the ground. This deeper cold layer allows the liquid drop sufficient time to completely refreeze into an ice pellet before reaching the surface. The key difference is that sleet arrives as a solid, while freezing rain arrives as a liquid that turns solid only upon contact.
Factors Influencing Surface Cooling
Several environmental variables work together to drive the surface temperature below the air temperature, creating the conditions for freezing rain. Wind speed plays a significant role through a process called evaporative cooling. As wind moves across a wet surface, it increases the rate at which water evaporates, and this phase change requires heat energy to be drawn directly from the surface, dropping its temperature.
Low humidity also enhances this evaporative cooling effect, allowing more moisture to transition into vapor and pull more heat from the surface material. This means that a light rain falling in dry, windy conditions can lead to freezing more readily than in still, saturated air. The specific material composition of the surface is also a factor.